Concrete slabs are made by providing a formwork that supports wet concrete until it has achieved sufficient strength to support itself and any imposed loads. Reinforcing steel is positioned on the formwork via chairs that set the required depth to finished concrete surfaces. The most conventional formwork consists of plywood boards, load distributing timber beams and braced structural props. The construction and removal of these elements has a high degree of risk. Stripping of formwork from a cured concrete slab involves progressively removing props, beams and boards from overhead and therefore serious consequence in the event of any incident. Whilst props, beams and formwork are in place during concrete cure, it is impractical to commence following trade work on that level. This constrains the speed at which construction of the building can occur.
Re-useable steel formwork and decking are known and can provide better quality and efficiency over plywood, but can be more expensive and are not as easily suited to complicated forms and levels.
Any reusable formwork system has a finite lifespan. Typically, plywood form boards would be used five to ten times before they would need to be replaced. Any damage and surface irregularity translates directly into the quality of finish on the concrete surface being formed. This becomes important where certain concrete class finish levels are required.
The most sophisticated current formwork systems include the use of a self-supporting deck which spans one-way between intermediate concrete or steel composite beams or load bearing walls. These decks are left in place and are therefore known as lost-formwork. Lost formwork systems include steel decking, fibre cement sheets, pre-cast concrete panels or plates (often with steel reinforcement exposed to engage with the concrete and reinforcement being placed on top). When there is an engagement between the concrete and a lost formwork decking, it can act in a composite manner thereby improving the load bearing capacity and deflection performance of the overall floor section.
Benefits of lost formwork systems include:
1. Support for wet concrete minimizing propping and beam support underneath the floor—materials handling improvements and better OH&S outcomes;
2. Composite construction results in greater strength;
3. Reduction in steel reinforcement required in the slab, e.g., steel decking formwork can act as bottom reinforcement in the slab, so only top steel is required;
4. Labor savings; and
5. Speed—no stripping of formwork—and can progress with works on level below earlier than with an off-form concrete system.
Composite metal decking is best employed laid over the top of steel beams. The resulting floor depth is the depth of the concrete floor element plus the beam. It is advantageous to minimize the depth of the floor including the beam, for example to minimize the overall height of the building or to maximize the number of floors within a building envelope, so this is not always the best solution. Composite metal decking can be placed on the bottom flange of the beam so that the beam is encased in concrete. Typically this would be a fabricated I-beam with a wider bottom flange. Another advantage of this method is the encasement of the full beam with concrete except for the underside of the bottom flange. The beam has a better capacity to resist fire without additional protection in that configuration and, if additional protection is required, it is to one surface only. However, the trade-off is an increase in the volume of concrete required for the floor with a consequent increase in mass with consequently heavier structural support members and footings. A better solution for composite metal decking is to fix to support angles fabricated part-way up the web of the beam. This encases the top of the beam while reducing the volume of concrete required for the floor. The trade-off in this case is additional fabrication work on the beam, reduced inherent fire performance with full three-sided fire protection required.
When lightweight pre-cast concrete panels, for example Hollowcore or AAC, are used as the lost formwork and placed on the bottom flange of a steel I-beam, the benefit of reducing the overall depth of the floor is achieved while also keeping the volume of concrete required to complete the floor to a minimum. These panels are less likely to need propping during the concrete pour and initial set due to their inherent structural capacity; in fact they are rarely designed to work compositely. The trade-off is that, while lighter in mass than concrete, they are heavier than composite metal decking, do not act as efficiently as a heat sink under fire conditions as normal poured concrete placed on the bottom flange, and have significantly higher cost than composite metal decking.
Void formers of polystyrene or other similar material incorporated on the top of sheet steel lost formwork can minimize the required volume of concrete but the process is complicated, adds significant cost and does not add to the structural capacity of the sheeting; in fact, it prevents the sheets from acting compositely with the concrete unless there are channels left for concrete to engage with the profiles, which defeats the purpose.
Lost formwork decking must be able to support the load of wet concrete as it's poured, together with point loading from the people laying and finishing it. Sheet steel lost formwork must provide the full load bearing capacity under that construction loading condition, that is, it must be a self-supporting metal deck. Consequently, the material content in the decking will be high relative to a composite metal deck which requires propping to handle that construction loading condition, but once the concrete has cured, it has sufficient strength to resist in-service loading conditions on its full span. When composite metal decking is propped, it is done in line so as to break up the full span between end supports. This means that beams must still be used to provide an even support. As previously discussed, removing temporary support beams from overhead represents a significant safety risk.
Sheet steel lost formwork and composite metal decks must be secured to the supports by self-drilling screws, spot welds, pins, some other mechanical means, or integrally with shear lugs, if the steel beam is to be composite with the slab, in order to prevent movement during construction from wind, human traffic and impacts from the concrete pour. Metal decking comes in sheets around 300 to 1100 mm wide and are installed individually on site once all the support beams are in place. Metal decking also requires void closers to be installed to prevent flow of wet concrete out of the floor deck to the level below.
Ceilings are usually suspended underneath concrete slabs via suspension rods, ties or straps that are anchored into the concrete. Installing anchors by drilling into concrete causes dust that is a health risk to the installer if inhaled. On sheet steel lost formwork, clips or anchoring mechanisms that work within the deck profile avoid this, but add to cost.
Pre-cast floor panels are craned into position one single unit at a time and this imposes a large constraint on crane time and its servicing of other areas on the site. Packs of composite metal decking can be delivered to the level in a single crane lift, but then must be installed manually.
Composite steel and concrete beams rely on shear lugs 90+ mm long with flanged heads welded to the top of the beams. This requires a fairly thick cover of concrete over the beam, going against the principle of reducing overall floor depth and mass. It is a process usually done on-site by welding through the composite metal decking, which, if it is galvanized as is usually the case, will release harmful gases.
The present invention seeks to provide a building floor structure and process for forming the same which will overcome or substantially ameliorate at least some of the deficiencies of the prior art, or to at least provide an alternative.
It is to be understood that, if any prior art information is referred to herein, such reference does not constitute an admission that the information forms part of the common general knowledge in the art, in Australia or any other country.